Needle Gauge Converter

Three completely unrelated needle sizing systems in one place, because a search for “needle gauge” returns all three mixed together: hypodermic needles on the Birmingham wire gauge, sewing machine needles as the Nm/Singer pair, and knitting needles in metric, US and old UK sizes. Two of the three number backwards.

Two tables and one definition

Hypodermic needles: the Birmingham gauge

Sewing machine needles: Nm over Singer

Knitting needles, and the wire gauge hiding in the UK column

Three systems, one word

Three needles, three scales, and two of them run backwards

Search for “needle gauge” and you get three unrelated answers at once, because three trades independently numbered their needles and none of them consulted the others. This page keeps them apart deliberately, and puts all three diameters on one chart, because seeing the ranges together is the fastest way to know which system you are in.

Hypodermic needles use the Birmingham gauge, also called the Stubs iron wire gauge, and that single historical fact explains the thing everyone finds counterintuitive: a higher number is a thinner needle. A 21G is 0.819 mm across the outside, a 25G is 0.514 mm, a 30G is 0.311 mm. The dimensions and their tolerances are specified by ISO 9626, and the colour of the hub by ISO 6009 — two different standards, routinely confused. Both are copyrighted, so this page cites them and takes its nominal diameters from a published Birmingham gauge table that a second, independent publisher confirms at every gauge it prints. Tolerances are not reproduced at all. And to be explicit: this is a dimensional reference and nothing else. Nothing here bears on clinical choice of needle, and the flow column is Poiseuille arithmetic on a nominal bore, not a measurement of anything.

Sewing machine needles are the honest ones. A packet says 80/12, and both numbers are live: 80 is the European Nm designation, fixed in 1942, and it is literally the shaft diameter in hundredths of a millimetre — so an 80 is 0.80 mm. 12 is the American (Singer) number, an index. The two are printed together precisely because neither trade would give up its own scale. There is a tidy relation, Singer = Nm/5 − 4, and it is exact from 60/8 all the way to 110/18 — and then it stops: Nm 120 is paired with Singer 19, not 20. Two independent charts agree on that, so the table wins and the rule is a shortcut with a documented end.

Knitting needles are where the trap is, and where the best fact on the page is. Three scales are in circulation: metric millimetres, a US number that runs up with diameter, and an old UK/Canadian number that runs down. A 4 mm needle is a US 6 and a UK 8; a 6 mm needle is a US 10 and a UK 4. So a British pattern calling for a number 8 and an American one calling for a number 8 are asking for needles two millimetres apart. The reason the UK numbers reverse is that they are Standard Wire Gauge numbers — the same scale, counting the draw plates the wire passed through, that gives hypodermic needles their backwards numbering. This page proves it rather than asserting it: the table above prints the actual SWG diameter for each old UK number, read out of this site’s own wire gauge record, and it tracks the metric size a shop sells to within 0.13 mm at every size from UK 13 to UK 2. The modern metric sizes are the old wire gauge, rounded to a convenient quarter of a millimetre.

What to do with all this. Write the diameter in millimetres. It is the one number all three systems can be checked against, it is unambiguous across borders, and for knitting it is what modern patterns use anyway. Where you must use a number, say the system in the same breath — “US 8”, “old UK 8”, “21 gauge”, “Nm 80”. For the wire gauge that underlies two of these three scales, see the wire gauge converter; for the yarn that goes on the knitting needles, the yarn count converter has the same kind of inversion problem in a worse form. This page does the arithmetic. Where the answer carries a consequence — a dose, a structural load, a legal area, a medical measure — confirm it against the authority that governs it before relying on it.

Frequently asked questions

Related calculators

References

1. Wikipedia, Birmingham gauge (read 26 September 2026) — the hypodermic needle table used here: gauge 7 to 34, outer and regular-wall inner diameter in inches and millimetres, and the ISO 6009 colour for each. It also records what this page repeats: the Birmingham gauge is the Stubs iron wire gauge, so a higher number is a THINNER needle, and thin-wall needles share the outer diameters but have larger bores.
2. Darwin Microfluidics, Syringe Needle Gauge Table (read 26 September 2026) — the independent check on the needle dimensions. It agrees exactly with the table above at all ten gauges it publishes: 14, 16, 18, 20, 21, 22, 23, 25, 27 and 30.
3. ISO 9626, Stainless steel needle tubing for the manufacture of medical devices — Requirements and test methods. This is the standard that fixes hypodermic needle tubing dimensions and their tolerances. It is copyrighted and none of its tables is reproduced here; the nominal diameters on this page come from the published Birmingham gauge table named below, and the tolerances are not given at all.
4. ISO 6009, Hypodermic needles for single use — Colour coding for identification. The colour of a needle hub is set by this standard and not by ISO 9626, which is a distinction worth keeping straight. Copyrighted; cited by number, with the colour names taken from the published table named below.
5. Wikipedia, Sewing machine needle (read 26 September 2026): the European designation, established in 1942, is the diameter of the needle in hundredths of a millimetre at a non-reinforced point above the scarf; the American (Singer) number is the second figure on the packet. Its table pairs 60/8 through 130/21.
6. American Sewing Guild / sewing.org, guideline 22.115, Sewing Machine Needle Charts (read 26 September 2026) — the second source for the pairings, agreeing at every one from 60/8 to 120/19, including the 120/19 where the arithmetic rule stops working.
7. Vogue Knitting, Knitting Needle Conversion Chart (read 26 September 2026): 22 rows of US size, metric millimetres and old UK size.
8. A Yarn Story, Knitting Needle Conversion Chart: US, UK & Metric Sizes (read 26 September 2026) — the second knitting chart. It agrees with Vogue’s at every row they share except the metric size of a US 19, which it gives as 15 mm where Vogue gives 16 mm, and it notes that the 12 mm needle is sometimes sold as 12.75 mm.
9. Wikipedia, Knitting needle (read 26 September 2026), for why the old UK numbers run backwards: they are a wire gauge, “based on the number of increasingly fine dies that the wire had to be drawn through”, so a thinner needle carries a bigger number.
10. BS 3737:1964, Specification for wire gauges, British Standards Institution — withdrawn, and copyrighted. Cited by number because the old UK knitting needle numbers ARE Standard Wire Gauge numbers; the SWG diameters this page checks them against are the ones already implemented in CalcEngines’ own wire gauge converter, from published tables rather than from the standard.
11. CalcEngines Converters, wire-gauge-converter — the SWG diameters this batch checks the old UK knitting needle numbers against were read out of that shipped record rather than re-entered, so the two pages cannot drift apart.